Monday night's episode of Supergirl gave viewers a healthy dose of '90s nostalgia, with Lois & Clark: The New Adventures of Superman's Teri Hatcher and Hercules' Kevin Sorbo both guest starring. And while it was fun to see the two appear as Daxamite royalty — and Mon-El's parents — the real excitement came at the end of the episode with the appearance of the Music Meister, played by Glee alum Darren Criss.

Ever since The Flash joined Arrow on the CW, crossovers have been commonplace for the network's DC television series. But on Tuesday night's episode of The Flash, viewers will be treated to a crossover unlike any they have seen before. By now, you probably know that Supergirl's Kara Danvers and The Flash's Barry Allen will be uniting in a musical!

While the crossover will primarily take place on The Flash, the final moments of Monday night's Supergirl set up the action, with the Music Meister allowing himself to be captured by the DEO, all so he could render Supergirl unconscious, and escape to Earth-1. There was also a bit of foreshadowing early in the episode, with Kara and Mon-El discussing romantic musicals.

It is not yet entirely clear why the Music Meister wants to travel to Earth-1, but it certainly does appear that he intends on confronting Barry Allen, and we know from the promos for Tuesday night's The Flash, that he will render the Fastest Man Alive in the same comatose state as Supergirl.

In season three, episode 17 of The Flash, titled "Duet," Mon-El and J'onn J'onzz will travel to Earth-1 with an unconscious Supergirl, with the hope that Team Flash can wake her up. It appears, though, that Team Flash will not be able to accomplish that feat, which is a problem, as Barry will quickly find himself in the same unconscious state. And the only way to wake up will be to sing and dance.

For more on Tuesday night's musical The Flash, here is the official synopsis for "Duet":

Barry (Grant Gustin) and team are surprised when Mon-El (guest star Chris Wood) and Hank Henshaw (guest star David Harewood) arrive on their Earth carrying a comatose Supergirl (guest star Melissa Benoist) who was whammied by the Music Meister (guest star Darren Criss). Unable to wake her up, they turn to Team Flash to save her. However, the Music Meister surprises the Flash and puts him in a similar coma, one that Team Flash can't cure. Kara and Barry wake up without their powers in an alternate reality where life is like a musical and the only way to escape is by following the script, complete with singing and dancing, to the end.

Need just a little more to hold you over? Check out the extended promo for "Duet" below: